The present invention concerns an indexable cutting bit for the turning of a polygonal basic shape which is defined by an upper side of polygonal contour, a parallel lower side and peripherally extending side faces interconnecting the upper and lower sides. The intersection of the side faces at least with the upper side defines cutting edges which comprise main cutting edges and secondary cutting edges. The substantially polygonal contour shape is defined by a generally triangular basic shape which has cut-off corner regions, wherein the main cutting edges are defined by the sides of the basic shape and the secondary cutting edges are defined by the cut-off corners. Each secondary cutting edge includes at least one concave and at least one convex cutting edge portion.
Such cutting bits are known for example for turning tubular, annular or pot-shaped work materials. In that case it is often necessary to use two different indexable cutting plates for external and internal machining of such a cylindrical workpiece. Corresponding cutting bits which are known from the state of the art are shown in accompanying FIGS. 1 and 2. Those cutting bits in accordance with the state of the art are of a square basic shape, wherein main cutting edges are defined along the transition of a top side face to the peripheral side faces, while the cut-off corner regions define secondary cutting edges.
A first cutting bit which is shown in FIG. 1 has at the transition from a main cutting edge h1 to a secondary cutting edge n1 in the corner region firstly a convex transition b1 followed by a concave cutting edge portion d1 at which there is again a convex transition c1 to the other main cutting edge adjacent to the corner. The secondary cutting edge therefore substantially comprises two convex transitions to the main cutting edges and a concave cutting edge portion disposed therebetween.
The cutting bit shown in FIG. 2 is of substantially the same basic structure, that is to say a main cutting edge h2 along a side of the (square) polygonal basic shape and a secondary cutting edge n2 in a cut-off corner region, wherein that secondary cutting edge is again formed from the convex transition b2 of a main cutting edge to a concave portion d2 of the secondary cutting edge, whereupon there is again a convex transition c2 to the other adjacent main cutting edge. In this example however the radius R of the convex transition C2 to the other adjacent cutting edge is substantially larger than the radius of the transition b2 from the first main cutting edge to the concave region. The purpose of that configuration can best be understood by referring to FIG. 3.
FIG. 3 shows the two cutting bits already illustrated in FIGS. 1 and 2 in use on the wall of a hollow-cylindrical workpiece. The wall of the hollow-cylindrical workpiece is shown here hatched in section, wherein the relative dimensions and in particular the proportion of the wall which is to be cut away are not shown true to scale. When dealing with certain cylindrical workpieces it may for example be desirable for the outside wall of the hollow-cylindrical workpieces to involve a greater roughness depth than the inner wall, or vice-versa.
By way of example let the upper side S in FIG. 3 be the outside cylindrical surface of the hollow-cylindrical workpiece and the lower side S′ in FIG. 3 be the inside cylindrical surface. The upper cutting bit 1 brings the outside surface to its definitive dimension using its main cutting edge h1 and its secondary cutting edge n1, the main cutting volume being afforded by the main cutting edge while the secondary cutting edge performs a finishing operation. During the finishing operation of the upper bit 1, a relatively thin cutting is also removed by the convex transition c1 from the concave cutting edge portion d1 to the non-active main cutting portion h1′ which is of a relatively small radius of curvature. In the case of the lower cutting bit 2, the main cutting work is also performed by the main cutting edge h2 and the secondary cutting edge n2 performs a finishing operation in a similar manner to the upper cutting edge, in this case however by the transition c2 from the concave portion d2 of the secondary cutting edge to the adjacent, non-active main cutting edge h2′ along a relatively large radius of curvature R. In this way the inside surface becomes substantially smoother and involves a substantially smaller roughness depth than the outside surface, as is also desired on the basis of corresponding predetermining factors.
It will thus be appreciated that the machining of corresponding workpieces in which the inside surface and the outside surface are to involve different respective roughness depths necessarily requires the use of two different types of cutting bits, although the cutting bits are otherwise produced from the substantially identical square basic shape and only the corner regions have to be correspondingly modified. In that respect confusion by an operator in respect of the cutting bits intended for the inside and outside surfaces can relatively easily occur, especially as in practice those cutting bits are only about 1/10th of the size as illustrated in the accompanying figures. The differing radii of curvature in the corner regions are therefore to be perceived with the naked eye only when particular attention is paid in that respect.
This correspondingly requires the stock-keeping of two different cutting bits which have to be carefully distinguished, and the cutting bits also have to be carefully distinguished upon fitment or installation in corresponding tools or tool holders. Added to that is the fact that only a small part of each of the main cutting edges is used as an active cutting edge while the major part of the main cutting edge remains unused and the cutting bit is worn after being indexed by turning three times (a total of four bit positions), assuming that single-sided cutting bits are involved here.
In comparison with that state of the art an object of the present invention is to provide a cutting bit for turning, which permits simultaneous roughing and finishing machining with selectively different roughness depths.